1. Field of the Invention
The present invention relates to a sensor device used for an occupant protection system.
2. Description of Related Art
As shown in FIG. 5A, an occupant protection system includes a sensor device 120, and an electronic control unit 115 (ECU) connected to the sensor device 120 through a low level communication line VL and a high level communication line VH. When a voltage of the low line VL is constant, and when a voltage of the high line VH oscillates, radiation noise is generated from the high line VH.
When a differential voltage/current communication is performed between the low line VL and the high line VH, the radiation noise is decreased.
The sensor device 120 includes a communication part 117, a sensor part 118 and a capacitor 119. The communication part 117 communicates with the ECU 115, and the sensor part 118 is connected to the communication part 117. A power supply line and a ground line connect the communication part 117 and the sensor part 118, and the capacitor 119 is arranged between the power supply line and the ground line.
When a predetermined communication is performed between the ECU 115 and the communication part 117, the sensor device 120 is activated by electricity charged in a capacitor 121 located between the low line VL and the communication part 117. When the predetermined communication is in an idle state, the sensor device 120 is activated by electricity supplied from the ECU 115. The ECU 115 controls the sensor device 120 to perform analog-digital conversion relative to a signal output from the sensor part 118, and to send the converted data to the ECU 115 through the communication part 117.
As shown in FIG. 5B, a phase of a high level signal waveform VHs of the high line VH is made opposite from that of a low level signal waveform VLs of the low line VL, in a communication between the ECU 115 and the sensor device 120. Thus, the radiation noise is reduced, because the waveform VLs is canceled by the waveform VHs having the opposite phase.
JP-A-2005-277546 discloses a communication apparatus. Radiation noise generated from a communication line of the communication apparatus is similarly reduced.
However, a sensor device connected to an ECU is not grounded, in the communication apparatus. A circuit element of the sensor device is connected to the low line VL as a standard. Further, a conduction pattern of the sensor device is connected to the low line VL. If radiation noise is generated from the circuit element and the conduction pattern, an amplitude modulation (AM) band of an in-vehicle radio may be affected by the radiation noise.